Preparation of the complex:

Calculate the amount of the reagents required to obtain  about 2 g of the product -
 for this, use the formula of the anhydrous compound - NaMgAl(C₂O₄)₃
(Actually you may get hydrated crystals)

Dissolve the oxalic acid in about 50 ml of water in a beaker. Add NaOH (solid with a little water to wash it down).
Wait for the solution to become clear. Then add the aluminium sulphate and stir to dissolve. Now add the MgO and stir
till the solution becomes clear. Heat the solution on a hot plate and add the barium hydroxide. Continue heating for
about 10 min. Let the precipitate settle down as much as possible. Filter and collect the filtrate in a clean beaker.
Reduce the volume of the filtrate to about 10 ml by heating on a water bath. Cool and see if any crystals form.
If not concentrate again and cool. Collect the crystals and wash with small quantities of water. Dry between
fold of filter paper and sore in sample bottle. Try some modifications -  The aim is to get a good yield of well formed crystals.

NOTE: The solution should be saturated for the crystalisation to begin. Slow cooling usually gives larger crystals.